Add persistent default temp directory setting with web UI

- Add settings.json persistence (load/save) in backend
- Add GET/POST /api/settings endpoints for default_temp_dir
- Add Default Temp Directory config UI in Temporary Storage section
- Auto-fallback: jobs use saved default if no per-job temp_dir set
- Per-job Advanced Options placeholder syncs with saved default
- Add settings.json to .gitignore for per-machine config

.gitignore

@@ -12,6 +12,7 @@ temp
 server.log
 .uvicorn.pid
 jobs.json
+settings.json
 
 # OS
 .DS_Store